Golf ball

ABSTRACT

In a golf ball having a core of at least one layer and a cover encasing the core, the cover is formed of a resin composition composed primarily of a polyurethane or a polyurea. Letting HMa (N/mm 2 ) be the Martens hardness of the cover measured at a position 0.3 mm toward the center of the ball from the cover surface and ηIta (%) be the elastic recovery of the cover, HMa and ηIta satisfy the following conditions: (1) ηIta≥−1.16×HMa+79, (2) 10.0≤HMa≤38.0, and (3) 40≤ηIta≤80. The ball has an excellent spin performance on approach shots and an excellent scuff resistance.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2018-237317 filed in Japan on Dec. 19, 2018, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a golf ball composed of two or more pieces, including a core and a cover. More specifically, the invention relates to a golf ball having both an excellent spin performance on approach shots and an excellent scuff resistance.

BACKGROUND ART

The chief characteristic demanded of golf balls is an increased distance, although other desired properties include an ability for the ball to stop well on approach shots and scuff resistance. Many golf balls endowed with a good flight on shots with a driver and a good receptivity to backspin on approach shots have hitherto been developed. In addition, golf ball cover materials possessing a high resilience and a good scuff resistance have been developed.

Today, urethane resin materials are often used in place of ionomeric resin materials as the cover material, especially in golf balls for professional golfers and skilled amateur golfers. However, professional golfers and skilled amateur golfers desire golf balls which have an even better controllability on approach shots, and so further improvement is sought even among cover materials in which a urethane resin material serves as the base resin. JP-A 2017-113220 discloses, as a cover material that endows the ball with excellent controllability around the green when played with a short iron such as a sand wedge and that can also extend the distance traveled by the ball on shots with a driver, a golf ball resin material which includes a specific styrenic thermoplastic elastomer and a thermoplastic resin having on the molecule either styrene monomer units or diene monomer units. Also, JP-A 2016-119946 discloses a resin material for golf balls that is composed primarily of a styrene-butadiene-styrene block copolymer and provides the ball with excellent controllability when hit around the green with a short iron such as a sand wedge.

However, although such golf ball resin materials do provide the ball with a high spin rate on approach shots and a good controllability, the scuff resistance is sometimes inadequate. Accordingly, there exists a desire to increase the controllability on approach shots and the scuff resistance even further than in the golf balls that have hitherto been disclosed in the art.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a golf ball which has an excellent spin performance on approach shots and also has an excellent scuff resistance.

As a result of extensive investigations, we have discovered that, in a golf ball having a core and a cover, when a resin composition containing a polyurethane or a polyurea as the chief component is used as the cover material and the golf ball is produced in such a way that, letting HMa (N/mm²) be the Martens hardness of the cover measured at a position 0.3 mm toward the center of the ball from the cover surface and ηIta (%) be the elastic recovery of the cover, HMa and ηIta satisfy the specific formula ηIta≥−1.16×HMa+79 (with the proviso that 10.0≤HMa≤38.0 and 40≤ηIta≤80), the golf ball has a higher spin rate on approach shots and a good scuff resistance.

Accordingly, the invention provides a golf ball having a core of at least one layer and a cover encasing the core, wherein the cover is formed of a resin composition composed primarily of a polyurethane or a polyurea and, letting HMa (N/mm²) be the Martens hardness of the cover measured at a position 0.3 mm toward the center of the ball from the cover surface and ηIta (%) be the elastic recovery of the cover, HMa and ηIta satisfy conditions (1) to (3) below:

ηIta≥−1.16×HMa+79  (1),

10.0≤HMa≤38.0  (2), and

40≤ηIta≤80  (3).

In a preferred embodiment of the golf ball of the invention, the Martens hardness of the cover (HMa) is from 11.0 to 24.0 N/mm².

In another preferred embodiment, the elastic recovery of the cover (ηIta) is from 50 to 75%.

In yet another preferred embodiment, the cover has a thickness of from 0.4 to 3.0 mm.

In a further preferred embodiment, the cover is formed of a thermoplastic composition containing a polyurethane, a polyurea or a mixture thereof.

In a still further preferred embodiment, a coating is formed on a surface of the cover.

In this preferred embodiment, letting HMb (N/mm²) be the Martens hardness of the coating and letting ηItb (%) be the elastic recovery of the coating, HMb and ηItb may satisfy conditions (4) and (5) below:

HMb≤20.0  (4), and

52≤ηItb≤82  (5).

Advantageous Effects of the Invention

The golf ball of the invention has an excellent spin performance on approach shots and also has an excellent scuff resistance.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic cross-sectional view of a golf ball according to an embodiment of the invention.

FIG. 2 is a graph showing the relationship between the Martens hardness HMa and the elastic recovery ηIta of the cover in the Examples of the invention and in the Comparative Examples.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the appended diagrams.

The golf ball of the invention has a core of at least one layer and a cover.

The core may be formed using a known rubber material as the base material. Known base rubbers, such as natural rubber or synthetic rubber, may be used as the base rubber. More specifically, the use of polybutadiene, especially cis-1,4-polybutadiene having a cis structure content of at least 40%, is recommended. If desired, natural rubber, polyisoprene rubber, styrene-butadiene rubber or the like may be used together with the foregoing polybutadiene in the base rubber.

The polybutadiene may be synthesized with a metal catalyst, such as a neodymium or other rare-earth catalyst, a cobalt catalyst or a nickel catalyst.

Co-crosslinking agents such as unsaturated carboxylic acids and metal salts thereof, inorganic fillers such as zinc oxide, barium sulfate and calcium carbonate, and organic peroxides such as dicumyl peroxide and 1,1-bis(t-butylperoxy)cyclohexane may be included in the base rubber. If necessary, commercial antioxidants and the like may also be suitably added.

In this invention, the cover is formed of a resin composition which includes a polyurethane or a polyurea. In this resin composition, a polyurethane, a polyurea or a mixture thereof accounts for a proportion of the overall resin composition which, from the standpoint of fully imparting the scuff resistance of the urethane resin, is generally at least 50 wt %, preferably at least 60 wt %, more preferably at least 80 wt %, and most preferably at least 90 wt %.

The above polyurethane and polyurea are described below.

Polyurethane

The polyurethane has a structure which includes soft segments composed of a polymeric polyol (polymeric glycol) that is a long-chain polyol, and hard segments composed of a chain extender and a polyisocyanate. Here, the polymeric polyol serving as a starting material may be any that has hitherto been used in the art relating to polyurethane materials, and is not particularly limited. This is exemplified by polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin polyols, conjugated diene polymer-based polyols, castor oil-based polyols, silicone-based polyols and vinyl polymer-based polyols. Specific examples of polyester polyols that may be used include adipate-type polyols such as polyethylene adipate glycol, polypropylene adipate glycol, polybutadiene adipate glycol and polyhexamethylene adipate glycol; and lactone-type polyols such as polycaprolactone polyol. Examples of polyether polyols include poly(ethylene glycol), poly(propylene glycol), poly(tetramethylene glycol) and poly(methyltetramethylene glycol). Such long-chain polyols may be used singly, or two or more may be used in combination.

The long-chain polyol preferably has a number-average molecular weight in the range of 1,000 to 5,000. By using a long-chain polyol having a number-average molecular weight in this range, golf balls made with a polyurethane composition that have excellent properties, including a good rebound and a good productivity, can be reliably obtained. The number-average molecular weight of the long-chain polyol is more preferably in the range of 1,500 to 4,000, and even more preferably in the range of 1,700 to 3,500.

Here and below, “number-average molecular weight” refers to the number-average molecular weight calculated based on the hydroxyl value measured in accordance with JIS-K1557.

The chain extender is not particularly limited; any chain extender that has hitherto been employed in the art relating to polyurethanes may be suitably used. In this invention, low-molecular-weight compounds with a molecular weight of 2,000 or less which have on the molecule two or more active hydrogen atoms capable of reacting with isocyanate groups may be used. Of these, preferred use can be made of aliphatic diols having from 2 to 12 carbon atoms. Specific examples include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. Of these, the use of 1,4-butylene glycol is especially preferred.

Any polyisocyanate hitherto employed in the art relating to polyurethanes may be suitably used without particular limitation as the polyisocyanate. For example, use can be made of one or more selected from the group consisting of 4,4′-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylene diisocyanate, xylylene diisocyanate, 1,5-naphthylene diisocyanate, tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, norbornene diisocyanate, trimethylhexamethylene diisocyanate, 1,4-bis(isocyanatomethyl)cyclohexane and dimer acid diisocyanate. However, depending on the type of isocyanate, crosslinking reactions during injection molding may be difficult to control.

The ratio of active hydrogen atoms to isocyanate groups in the polyurethane-forming reaction may be suitably adjusted within a preferred range. Specifically, in preparing a polyurethane by reacting the above long-chain polyol, polyisocyanate and chain extender, it is preferable to use the respective components in proportions such that the amount of isocyanate groups included in the polyisocyanate per mole of active hydrogen atoms on the long-chain polyol and the chain extender is from 0.95 to 1.05 moles.

The method of preparing the polyurethane is not particularly limited. Preparation using the long-chain polyol, chain extender and polyisocyanate may be carried out by either a prepolymer process or a one-shot process via a known urethane-forming reaction. Of these, melt polymerization in the substantial absence of solvent is preferred. Production by continuous melt polymerization using a multiple screw extruder is especially preferred.

It is preferable to use a thermoplastic polyurethane material as the polyurethane. The thermoplastic polyurethane material may be a commercial product, examples of which include those available under the trade name Pandex from DIC Covestro Polymer, Ltd., and those available under the trade name Resamine from Dainichiseika Color & Chemicals Mfg. Co., Ltd.

Polyurea

The polyurea is a resin composition composed primarily of urea linkages formed by reacting (i) an isocyanate with (ii) an amine-terminated compound. This resin composition is described in detail below.

(i) Isocyanate

The isocyanate is preferably one that is used in the prior art relating to polyurethanes, but is not particularly limited. Use may be made of isocyanates similar to those mentioned above in connection with the polyurethane material.

(ii) Amine-Terminated Compound

An amine-terminated compound is a compound having an amino group at the end of the molecular chain. In this invention, the long-chain polyamines and/or amine curing agents shown below may be used.

A long-chain polyamine is an amine compound which has on the molecule at least two amino groups capable of reacting with isocyanate groups, and which has a number-average molecular weight of from 1,000 to 5,000. In this invention, the number-average molecular weight is more preferably from 1,500 to 4,000, and even more preferably from 1,900 to 3,000. Examples of such long-chain polyamines include, but are not limited to, amine-terminated hydrocarbons, amine-terminated polyethers, amine-terminated polyesters, amine-terminated polycarbonates, amine-terminated polycaprolactones, and mixtures thereof. These long-chain polyamines may be used singly, or two or more may be used in combination.

An amine curing agent is an amine compound which has on the molecule at least two amino groups capable of reacting with isocyanate groups, and which has a number-average molecular weight of less than 1,000. In this invention, the number-average molecular weight is more preferably less than 800, and even more preferably less than 600. Specific examples of such amine curing agents include, but are not limited to, ethylenediamine, hexamethylenediamine, 1-methyl-2,6-cyclohexyldiamine, tetrahydroxypropylene ethylenediamine, 2,2,4- and 2,4,4-trimethyl-1,6-hexanediamine, 4,4′-bis(sec-butylamino)dicyclohexylmethane, 1,4-bis(sec-butylamino)cyclohexane, 1,2-bis(sec-butylamino)cyclohexane, derivatives of 4,4′-bis(sec-butylamino)dicyclohexylmethane, 4,4′-dicyclohexylmethanediamine, 1,4-cyclohexane bis(methylamine), 1,3-cyclohexane bis(methylamine), diethylene glycol di(aminopropyl) ether, 2-methylpentamethylenediamine, diaminocyclohexane, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, propylenediamine, 1,3-diaminopropane, dimethylaminopropylamine, diethylaminopropylamine, dipropylenetriamine, imidobis(propylamine), monoethanolamine, diethanolamine, triethanolamine, monoisopropanolamine, diisopropanolamine, isophoronediamine, 4,4′-methylenebis(2-chloroaniline), 3,5-dimethylthio-2,4-toluenediamine, 3,5-dimethylthio-2,6-toluenediamine, 3,5-diethylthio-2,4-toluenediamine, 3,5-diethylthio-2,6-toluenediamine, 4,4′-bis(sec-butylamino)diphenylmethane and derivatives thereof, 1,4-bis(sec-butylamino)benzene, 1,2-bis(sec-butylamino)benzene, N,N′-dialkylaminodiphenylmethane, N,N,N′,N′-tetrakis(2-hydroxypropyl)ethylenediamine, trimethylene glycol di-p-aminobenzoate, polytetramethylene oxide di-p-aminobenzoate, 4,4′-methylenebis(3-chloro-2,6-diethyleneaniline), 4,4′-methylenebis(2,6-diethyl aniline), m-phenylenediamine, p-phenylenediamine and mixtures thereof. These amine curing agents may be used singly or two or more may be used in combination.

(iii) Polyol

Although not an essential ingredient, in addition to the above-described components (i) and (ii), a polyol may also be included in the polyurea. The polyol is not particularly limited, but is preferably one that has hitherto been used in the art relating to polyurethanes. Specific examples include the long-chain polyols and/or polyol curing agents mentioned below.

The long-chain polyol may be any that has hitherto been used in the art relating to polyurethanes. Examples include, but are not limited to, polyester polyols, polyether polyols, polycarbonate polyols, polyester polycarbonate polyols, polyolefin-based polyols, conjugated diene polymer-based polyols, castor oil-based polyols, silicone-based polyols and vinyl polymer-based polyols. These long-chain polyols may be used singly or two or more may be used in combination.

The long-chain polyol has a number-average molecular weight of preferably from 1,000 to 5,000, and more preferably from 1,700 to 3,500. In this average molecular weight range, an even better resilience and productivity are obtained.

The polyol curing agent is preferably one that has hitherto been used in the art relating to polyurethanes, but is not subject to any particular limitation. In this invention, use may be made of a low-molecular-weight compound having on the molecule at least two active hydrogen atoms capable of reacting with isocyanate groups, and having a molecular weight of less than 1,000. Of these, the use of aliphatic diols having from 2 to 12 carbon atoms is preferred. Specific examples include 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and 2,2-dimethyl-1,3-propanediol. The use of 1,4-butylene glycol is especially preferred. The polyol curing agent has a number-average molecular weight of preferably less than 800, and more preferably less than 600.

A known method may be used to produce the polyurea. A prepolymer process, a one-shot process or some other known method may be suitably selected for this purpose.

In the cover-forming resin composition, from the standpoint of the spin properties and scuff resistance that can be obtained in the golf ball, the material hardness on the Shore D scale is preferably not more than 65, more preferably not more than 60, and even more preferably not more than 55. From the standpoint of the moldability, the lower limit in the material hardness on the Shore D scale is preferably at least 25, and more preferably at least 30.

In addition to the above polyurethane or polyurea resin component, other resin materials may also be included. The purpose for doing so is, for example, to further improve the flowability of the golf ball resin composition and to increase various properties of the golf ball such as rebound and scuff resistance.

The other resin materials may be selected from among polyester elastomers, polyamide elastomers, ionomeric resins, ethylene-ethylene/butyl ene-ethylene block copolymers and modified forms thereof, polyacetals, polyethylenes, nylon resins, methacrylic resins, polyvinyl chlorides, polycarbonates, polyphenylene ethers, polyarylates, polysulfones, polyethersulfones, polyetherimides and polyamideimides. These may be used singly or two or more may be used together.

In addition, an isocyanate compound may be included in the above cover-forming resin composition. This is because the reaction of a polyurethane or a polyurea with an isocyanate compound can further increase the scuff resistance of the resin composition. Moreover, the plasticizing effect of the isocyanate can increase the flowability of the resin composition and improve the moldability.

Any isocyanate compound employed in conventional polyurethanes may be used without particular limitation as the above isocyanate compound. For example, aromatic isocyanate compounds that may be used include 2,4-toluene diisocyanate, 2,6-toluene diisocyanate and mixtures of both, 4,4-diphenylmethane diisocyanate, m-phenylene diisocyanate and 4,4′-biphenyl diisocyanate. Use can also be made of the hydrogenated forms of these aromatic isocyanate compounds, such as dicyclohexylmethane diisocyanate. Other isocyanate compounds that may be used include aliphatic diisocyanates such as tetramethylene diisocyanate, hexamethylene diisocyanate (HDI) and octamethylene diisocyanate; and alicyclic diisocyanates such as xylene diisocyanate. Further examples of isocyanate compounds that may be used include blocked isocyanate compounds obtained by reacting the isocyanate groups on a compound having two or more isocyanate groups on the ends with a compound having active hydrogens, and uretdiones obtained by the dimerization of isocyanate.

The amount of the above isocyanate compounds included per 100 parts by weight of the polyurethane or polyurea resin serving as the base resin is preferably at least 0.1 part by weight, and more preferably at least 0.5 part by weight. The upper limit is preferably not more than 30 parts by weight, and more preferably not more than 20 parts by weight. When too little is included, a sufficient crosslinking reaction may not be obtained and an increase in the properties may not be observable. On the other hand, when too much is included, discoloration over time due to heat and ultraviolet light may increase, or problems may arise such as a loss of thermoplasticity or a decline in resilience.

In addition, depending on the intended use, optional additives may be suitably included in the above resin composition. For example, various types of additives, such as inorganic fillers, organic staple fibers, reinforcing agents, crosslinking agents, pigments, dispersants, antioxidants, ultraviolet absorbers and light stabilizers, may be added. When such additives are included, the amount thereof, per 100 parts by weight of the base resin, is preferably at least 0.1 part by weight, and more preferably at least 0.5 part by weight, but preferably not more than 10 parts by weight, and more preferably not more than 4 parts by weight.

The above cover-forming resin composition may be prepared by mixing together the ingredients of the resin material using any of various types of mixers, such as a kneading-type single-screw or twin-screw extruder, a Banbury mixer, a kneader or a Labo Plastomill. Alternatively, the ingredients may be mixed by dry blending at the time that the resin composition is to be injection molded. In addition, when an isocyanate compound is used, it may be incorporated at the time of resin mixture using various types of mixers, or a resin masterbatch already containing the isocyanate compound may be separately prepared and the various components mixed together by dry blending at the time that the resin composition is to be injection molded.

The cover may be molded by, for example, feeding the above-described resin composition to an injection molding machine and injecting the molten resin composition over the core. In this case, the molding temperature varies depending on the type of polyurethane, polyurea or the like, but is generally in the range of 150 to 270° C.

In the golf ball of the invention, the cover has a hardness at a given position such that, letting HMa (N/mm²) be the Martens hardness of the cover measured at a position 0.3 mm toward the center of the ball from the cover surface and ηIta (%) be the elastic recovery of the cover, HMa and ηIta satisfy conditions (1) to (3) below:

ηIta≥−1.16×HMa+79  (1),

10.0≤HMa≤38.0  (2), and

40≤ηIta≤80  (3).

The Martens hardness HMa of the cover is, as noted above, from 10.0 to 38.0 N/mm². The lower limit is preferably at least 11.0 N/mm², and more preferably at least 12 N/mm². The upper limit is preferably not more than 36.0 N/mm², and more preferably not more than 34.0 N/mm². At a Martens hardness within the range of 10.0 to 38.0 N/mm², the ball has an excellent scuff resistance and an excellent spin performance on approach shots.

The Martens hardness can be measured with a nanohardness tester based on ISO 14577: 2002 (“Metallic materials—Instrumented indentation test for hardness and materials parameters”). This is a physical value determined by pressing an indenter into the object being measured while applying a load to the indenter, and is calculated as (indentation force [N])/(surface area of region to which pressure is applied [mm²]). Measurement of the Martens hardness may be carried out using, for example, the nanohardness tester available from Fischer Instruments under the product name Fischerscope HM2000. This instrument can, for example, measure the hardness of the cover while successively increasing the load in a stepwise manner. The nanohardness test conditions may be set to room temperature, 10 seconds, and an applied load of 50 mN.

When measuring the surface of the cover, because a coating, etc. has been formed on the cover surface, it is difficult to specify the surface hardness. Also, given that deep positions from the cover surface toward the center of the ball are influenced by the hardness of the adjacent layer, the Martens hardness inherent to the cover can be stably obtained at a position 0.3 mm from the cover surface toward the center of the ball.

The cover has a work recovery ηIta which, as mentioned above, is from 40 to 80%, and preferably from 50 to 75%. At an elastic work recovery in this range, the cover formed at the golf ball surface has a high self-repairing/recovering ability while maintaining a fixed hardness and elasticity, and is able to contribute to the excellent durability and scuff resistance of the ball. Moreover, in cases where the elastic recovery is low even though the Martens hardness is low, the ball has a good spin performance on approach shots but a poor scuff resistance. The method of measuring the elastic work recovery is described below.

The elastic work recovery is one parameter of the nanoindentation method for evaluating the physical properties of the cover, this being a nanohardness test method that controls the indentation load on a micro-newton (μN) order and tracks the indenter depth during indentation to a nanometer (nm) precision. In prior methods, only the size of the deformation (plastic deformation) mark corresponding to the maximum load could be measured. However, in the nanoindentation method, the relationship between the indentation load and the indentation depth can be obtained by continuous automated measurement. Hence, unlike in the past, there are no individual differences between observers when visually measuring a deformation mark under an optical microscope, thus enabling the physical properties of the cover to be measured reliably and to a high precision. Given the strong influence on the cover of a golf ball when the ball is struck by a driver or other golf club and the fact that this cover has a not inconsiderable influence on various golf ball properties, measuring the cover by the nanohardness test method and carrying out such measurement to a higher precision than in the past is a very effective method of evaluation.

The cover has a thickness of preferably at least 0.4 mm, more preferably at least 0.5 mm, and even more preferably at least 0.6 mm. The upper limit is preferably not more than 3.0 mm, and more preferably not more than 2.0 mm.

At least one intermediate layer may be interposed between the core and the cover. In this case, various types of thermoplastic resins used in golf ball cover stock may be used as the intermediate layer material, with the use of an ionomeric resin being especially preferred. A commercial product may be used as the ionomeric material.

In the golf ball of the invention, for reasons having to do with the aerodynamic performance, numerous dimples are provided on the surface of the outermost layer. The number of dimples formed on the surface of the outermost layer is not particularly limited. However, to enhance the aerodynamic performance and increase the distance traveled by the ball, this number is preferably at least 250, more preferably at least 270, even more preferably at least 290, and most preferably at least 300. The upper limit is preferably not more than 400, more preferably not more than 380, and even more preferably not more than 360.

In this invention, a coating layer is formed on the cover surface. A two-part curable urethane coating may be suitably used as the coating that forms this coating layer. Specifically, in this case, the two-part curable urethane coating is one that includes a base resin composed primarily of a polyol resin and a curing agent composed primarily of a polyisocyanate.

A known method may be used without particular limitation as the method for applying this coating onto the cover surface and forming a coating layer. Use can be made of a desired method such as air gun painting or electrostatic painting.

The thickness of the coating layer, although not particularly limited, is generally from 8 to 22 μm, and preferably from 10 to 20 μm.

Letting HMb (N/mm²) be the Martens hardness of the coating layer and letting ηItb (%) be the elastic recovery of the coating layer, it is preferable for HMb and ηItb to satisfy conditions (4) and (5) below:

HMb≤20.0  (4), and

52≤ηItb≤82  (5).

In the present invention, the coating layer has an increased self-repairing/recovering performance while retaining its hardness and elasticity. These properties, in combination with the makeup of the cover, contribute to the excellent durability and scuff resistance of the ball. In addition, the spin performance of the ball on approach shots also is even better.

The methods used to measure the Martens hardness and the elastic recovery of the above coating, aside from the particular conditions employed such as the applied load, are the same as those described above for measuring the Martens hardness and the elastic recovery of the cover.

The golf ball of the invention can be made to conform to the Rules of Golf for play. The inventive ball may be formed to a diameter which is such that the ball does not pass through a ring having an inner diameter of 42.672 mm and is not more than 42.80 mm, and to a weight which is preferably between 45.0 and 45.93 g.

EXAMPLES

The following Examples and Comparative Examples are provided to illustrate the invention, and are not intended to limit the scope thereof.

Examples 1 to 10, Comparative Examples 1 to 3

A core-forming rubber composition formulated as shown in Table 1 and common to all of the Examples except for Comparative Example 2 was prepared and then molded and vulcanized to produce a 38.6 mm diameter core.

TABLE 1 Rubber composition parts by weight cis-1,4-Polybutadiene 100 Zinc acrylate 27 Zinc oxide 4.0 Barium sulfate 16.5 Antioxidant 0.2 Organic peroxide (1) 0.6 Organic peroxide (2) 1.2 Zinc salt of pentachlorothiophenol 0.3 Zinc stearate 1.0

Details on the ingredients mentioned in Table 1 are given below.

-   Cis-1,4-Polybutadiene: Available under the trade name “BR 01” from     JSR Corporation -   Zinc acrylate: From Nippon Shokubai Co., Ltd. -   Zinc oxide: From Sakai Chemical Co., Ltd. -   Barium sulfate: From Sakai Chemical Co., Ltd. -   Antioxidant: Available under the trade name “Nocrac NS6” from Ouchi     Shinko Chemical Industry Co., Ltd. -   Organic peroxide (1): Dicumyl peroxide, available under the trade     name “Percumyl D” from NOF Corporation -   Organic peroxide (2): A mixture of     1,1-di(tert-butylperoxy)cyclohexane and silica, available under the     trade name “Perhexa C-40” from NOF Corporation -   Zinc stearate: Available from NOF Corporation

Next, an intermediate layer-forming resin material common to all of the Examples was formulated. This intermediate layer resin material was a blend of 50 parts by weight of a sodium-neutralized ethylene-unsaturated carboxylic acid copolymer having an acid content of 18 wt % and 50 parts by weight of a zinc-neutralized ethylene-unsaturated carboxylic acid copolymer having an acid content of 15 wt % (for a combined amount of 100 parts by weight). This resin material was injection molded over a core having a diameter of 38.6 mm, thereby producing an intermediate layer-encased sphere having an intermediate layer with a thickness of 1.25 mm.

Next, the cover materials for the respective Examples and Comparative Examples shown below were injection molded over the intermediate layer-encased spheres in the amounts indicated in Table 2, thereby producing 42.7 mm diameter three-piece golf balls having a cover layer (outermost layer) with a thickness of 0.8 mm. Although not shown in the diagrams, dimples common to all the Examples and Comparative Examples were formed on the surface of the cover.

Preparation of Cover-Forming Resin Composition

The cover-forming resin compositions in the respective Examples and Comparative Examples are shown in Table 2 below.

TABLE 2 Comparative Example Example 1 2 3 4 5 6 7 8 9 10 1 2 3 Cover TPU 1 TPU 2 TPU 3 TPU 4 TPU 5 TPU 6 TPU 7 TPU 8 TPU 9 TPU 10 TPU 11 com- TPU 12 material mercial product Cover 53 50 48 47 45 44 43 40 40 39 36 — 56 hardness (Shore D) MDI * 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 3.5 0 — 7.0 (wt %) * Numbers given for MDI indicate the amount of MDI included in the various TPU products.

In Examples 1 to 10 and Comparative Examples 1 and 3, “TPU 1” to “TPU 12” are ether-type thermoplastic polyurethanes available from DIC Covestro Polymer, Ltd. under the trade name “Pandex,” and “MDI” is 4,4′-diphenylmethane diisocyanate (an isocyanate compound). The commercial product in Comparative Example 2 is the Titleist ProV1x (2017 model) golf ball (Acushnet), which has a urethane cover.

Martens Hardness of Cover

The golf ball in each Example was cut in half and, specifying a position on the ball cross-section that is located 0.3 mm from the surface of the cover toward the ball center, the Martens hardness HMa (N/mm²) at this place was measured using the nanohardness tester available from Fischer Instruments under the product name Fischerscope HM2000. The nanohardness measurement conditions were room temperature and an applied load of 50 mN/10 s.

Elastic Work Recovery of Cover

The elastic work recovery of the cover was measured using the nanohardness tester available from Fischer Instruments under the product name Fischerscope HM2000. The measurement conditions were room temperature and an applied load of 50 mN/10 s. The elastic work recovery was calculated as follows, based on the indentation work W_(elast) (Nm) due to spring-back deformation of the cover and on the mechanical indentation work W_(total) (Nm).

Elastic work recovery=W _(elast) /W _(total)×100(%)

FIG. 2 shows a plot of the Martens hardness HMa of the cover versus the elastic recovery ηIta of the cover in the Examples and Comparative Examples.

Formation of Coating Layer

Next, a coating formulated as shown in Table 3 below was applied with an air spray gun onto the surface of the cover (outermost layer) on which numerous dimples had been formed, thereby producing golf balls having a 15 μm-thick coating layer formed thereon. Coating Formulation A below was used in the golf balls in Examples 1 to 7, Examples 9 and 10 and Comparative Examples 1 and 3, and Coating Formulation B below was used in the golf ball in Example 8.

TABLE 3 Coating formulation (pbw) A B Base resin Polyol *1 100.0 Polyol *2 100.0 Ethyl acetate 60.0 100.0 Propylene glycol 40.0 40.0 monomethyl ether acetate Curing catalyst 0.03 0.03 Curing agent Isocyanurate form of 52.5 30.5 hexamethylene diisocyanate (1) Modified polyester of 46.8 hexamethylene diisocyanate (2) Ethyl acetate 47.5 42.7 Molar compounding ratio (NCO/OH) 1.08 1.08 Coating Formulation B (molar ratio of NCO): (1):(2) = 0.79:0.29

Synthesis Examples for Acrylic Polyol *1 and *2 in Table 3 are described below. Here, “parts” signifies parts by weight.

Acrylic Polyol Synthesis Example 1

A reactor equipped with a stirrer, a thermometer, a condenser, a nitrogen gas inlet and a dropping device was charged with 1,000 parts of butyl acetate and the temperature was raised to 100° C. under stirring. Next, a mixture consisting of 620 parts of a polyester-containing acrylic monomer (Placcel FM-3, from Daicel Chemical Industries, Ltd.), 317 parts of methyl methacrylate, 63 parts of 2-hydroxyethyl methacrylate and 12 parts of 2,2′-azobisisobutyronitrile was added dropwise over 4 hours. After the end of dropwise addition, the reaction was effected for 6 hours at the same temperature. Following reaction completion, 532 parts of butyl acetate and 520 parts of polycaprolactone diol (Placcel L205AL, from Daicel Chemical Industries, Ltd.) were charged and mixed in, giving a clear acrylic polyol resin solution (Polyol *1) having a solids content of 50%, a viscosity of 600 mPa·s (25° C.), a weight-average molecular weight of 70,000, and a hydroxyl value of 142 mgKOH/g (solids).

Acrylic Polyol Synthesis Example 2

A reactor equipped with a stirrer, a thermometer, a condenser, a nitrogen gas inlet and a dropping device was charged with 1,000 parts of butyl acetate and the temperature was raised to 100° C. under stirring. Next, a mixture consisting of 220 parts of a polyester-containing acrylic monomer (Placcel FM-3, from Daicel Chemical Industries, Ltd.), 610 parts of methyl methacrylate, 170 parts of 2-hydroxyethyl methacrylate and 30 parts of 2,2′-azobisisobutyronitrile was added dropwise over 4 hours. After the end of dropwise addition, the reaction was effected for 6 hours at the same temperature. Following reaction completion, 180 parts of butyl acetate and 150 parts of polycaprolactone diol (Placcel L205AL, from Daicel Chemical Industries, Ltd.) were charged and mixed in, giving a clear acrylic polyol resin solution (Polyol *2) having a solids content of 50%, a viscosity of 100 mPa·s (25° C.), a weight-average molecular weight of 10,000, and a hydroxyl value of 113 mgKOH/g (solids).

The ball diameter, ball deflection, Martens hardness and elastic recovery were measured as described below for the golf balls obtained in the respective Examples and Comparative Examples. The results are shown in Table 4.

Ball Diameter

The diameters at 15 random dimple-free areas on the surface of a ball were measured at a temperature of 23.9±1° C. and, using the average of these measurements as the measured value for a single ball, the average diameter for five measured balls was determined.

Ball Deflection

A ball was placed on a hard plate and the amount of deflection (mm) when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) was measured. The amount of deflection here refers in each case to the measured value obtained after holding the ball isothermally at 23.9° C.

Martens Hardness and Elastic Modulus of Ball (Coating)

With the apparatus used above to measure the Martens hardness of the cover, the golf ball was placed in a half cap, the spot to be measured was brought up to the peak and, using the base of a dimple as the measurement position, the focus was adjusted and measurement was carried out. The measurement conditions were as follows.

Temperature: about 23 to 24° C.

Load F: 0.3 mN

Loading time: 10 seconds

Number of measurements (N): 3

The elastic work recovery was calculated as follows, based on the indentation work W_(elast) (Nm) due to spring-back deformation of the coating and on the mechanical indentation work W_(total) (Nm).

Elastic work recovery=W _(elast) /W _(total)×100(%)

The spin performance on approach shots and the scuff resistance were evaluated as described below for the golf balls obtained in the respective Examples and Comparative Examples.

Spin Performance on Approach Shots

A sand wedge (SW) was mounted on a golf swing robot and the spin rate of the ball when struck at a head speed of 20 m/s was measured. The club used was the TourStage X-WEDGE (loft angle, 56°) manufactured by Bridgestone Sports Co., Ltd.

Scuff Resistance

The golf balls were held isothermally at 23° C. and five balls of each type were hit at a head speed of 33 m/s using as the club a pitching wedge mounted on a swing robot machine. The damage to the ball from the impact was visually rated based on the following 5-point scale.

5: No damage or substantially no apparent damage.

4: Damage is apparent but so slight as to be of substantially no concern.

3: Surface is slightly frayed.

2: Some fraying of surface or loss of dimples.

1: Dimples completely obliterated in places.

Comparative Example 2

In Comparative Example 2 as well, the spin performance of the golf ball on approach shots and the scuff resistance were evaluated in the same way as in the above Examples. The golf ball in Comparative Example 2 was a four-piece solid golf ball with a two-layer core, an intermediate layer and a cover that is available from the Acushnet Company under the product name “Titleist ProV1X, 2017 model.”

TABLE 4 Example 1 2 3 4 5 6 7 Cover HMa (N/mm²) 37.0 30.3 27.3 23.8 22.4 21.8 17.2 ηIta (%) 43 49 50 52 54 55 66 −1.16 × HMa + 79 36 44 47 51 53 54 59 Coating HMb (N/mm²) 13.7 13.7 13.7 13.7 13.7 13.7 13.7 ηItb (%) 78 78 78 78 78 78 78 Ball Core diameter (mm) 38.6 38.6 38.6 38.6 38.6 38.6 38.6 Core weight (g) 34.9 34.9 34.9 34.9 34.9 34.9 34.9 Intermediate layer 41.1 41.1 41.1 41.1 41.1 41.1 41.1 diameter (mm) Intermediate layer 40.6 40.6 40.6 40.6 40.6 40.6 40.6 weight (g) Diameter (mm) 42.70 42.70 42.70 42.70 42.70 42.70 42.71 Weight (g) 45.59 45.59 45.60 45.59 45.58 45.55 45.62 Deflection (mm) 2.70 2.71 2.71 2.71 2.71 2.71 2.72 Evaluation Spin rate on 6,039 6,184 6,198 6,204 6,232 6,310 6,346 approach shots (rpm) Scuff resistance 3.3 3.5 3.6 3.7 3.8 3.8 4.0 (score) Comparative Example Example 8 9 10 1 2 3 Cover HMa (N/mm²) 16.1 16.1 11.1 8.2 22.3 40.3 ηIta (%) 64 64 75 63 48 40 −1.16 × HMa + 79 60 60 66 68 53 32 Coating HMb (N/mm²) 21.5 13.7 13.7 13.7 13.7 13.7 ηItb (%) 68 78 78 78 78 78 Ball Core diameter (mm) 38.6 38.6 38.6 38.6 38.6 38.6 Core weight (g) 34.9 34.9 34.9 34.9 34.9 34.9 Intermediate layer 41.1 41.1 41.1 41.1 41.1 41.1 diameter (mm) Intermediate layer 40.6 40.6 40.6 40.6 40.6 40.6 weight (g) Diameter (mm) 42.71 42.71 42.71 42.71 42.70 42.70 Weight (g) 45.61 45.61 45.61 45.59 45.61 45.55 Deflection (mm) 2.72 2.72 2.73 2.73 2.32 2.69 Evaluation Spin rate on 6,220 6,371 6,546 6,541 6,236 5,847 approach shots (rpm) Scuff resistance 3.5 4.0 4.0 2.0 2.9 2.7 (score) The golf ball product in Comparative Example 2 was the Titleist ProV1x, 2017 model (from Acushnet).

As is apparent from the results in Table 4, the golf balls in Examples 1 to 10 according to this invention satisfied above conditions (1) to (3). As a result, these balls had a high spin rate on approach shots and a good controllability, in addition to which the scuff resistance was good.

By contrast, the golf ball in Comparative Example 1 did not satisfy above conditions (1) and (2) and so, although the spin rate on approach shots rose and the controllability was good, the scuff resistance was poor. The golf ball in Comparative Example 2 did not satisfy condition (1); hence, although the spin rate on approach shots rose and the controllability was good, the scuff resistance was poor. The golf ball in Comparative Example 3 did not satisfy condition (2), and so the spin rate on approach shots and the scuff resistance were both poor.

Japanese Patent Application No. 2018-237317 is incorporated herein by reference.

Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims. 

1. A golf ball comprising a core of at least one layer and a cover encasing the core, wherein the cover is formed of a resin composition composed primarily of a polyurethane or a polyurea and, letting HMa (N/mm²) be the Martens hardness of the cover measured at a position 0.3 mm toward the center of the ball from the cover surface and ηIta (%) be the elastic recovery of the cover, HMa and ηIta satisfy conditions (1) to (3) below: ηIta≥−1.16×HMa+79  (1), 10.0≤HMa≤38.0  (2), and 40≤ηIta≤80  (3).
 2. The golf ball of claim 1, wherein the Martens hardness of the cover (HMa) is from 11.0 to 24.0 N/mm².
 3. The golf ball of claim 1, wherein the elastic recovery of the cover (ηIta) is from 50 to 75%.
 4. The golf ball of claim 1, wherein the cover has a thickness of from 0.4 to 3.0 mm.
 5. The golf ball of claim 1, wherein the cover is formed of a thermoplastic composition containing a polyurethane, a polyurea or a mixture thereof.
 6. The golf ball of claim 1, wherein a coating is formed on a surface of the cover.
 7. The golf ball of claim 6 wherein, letting HMb (N/mm²) be the Martens hardness of the coating and letting ηItb (%) be the elastic recovery of the coating, HMb and ηItb satisfy conditions (4) and (5) below: HMb≤20.0  (4), and 52≤ηItb≤82  (5). 